1. Field of the Invention
The present invention relates to a signal modulation of direct sequence spread spectrum communication system and, particularly to a modulation apparatus of multicarrier direct sequence spread spectrum communication system.
2. Information Disclosure Statement
Code division multiple access (CDMA) method using Direct Sequence Spectrum Spread Communication method is currently used in digital cellular and personal mobile communication system. Next generation CDMA system of high data transmission rate must use wider frequency band. To use a wider frequency in CDMA signal transmission method, a single carrier and a multicarrier direct sequence spread spectrum communication methods can be used.
In the single carrier direct sequence spread spectrum communication method, one carrier is modulated by a spectrum spread signal obtained by combining a data with a PN sequence having a faster code chip generation rate than that used in narrow band direct sequence spread spectrum communication system. In the multicarrier direct sequence spread spectrum communication method, multiple disjoint carriers are modulated by a spectrum spread signal obtained by combining data and a sequence having the same code chip generation rate as that used in the narrow band direct sequence spread spectrum system. In the multicarrier system, since the channel characteristics is different for each frequency, a diversity gain can be obtained at the time of signal reception. Since there is no need of using a fast operating device in baseband multicarrier signal processing, electric power consumption can be reduced, and, further, since there is no need of using successive frequency bands in contrast to the case of using single frequency, the efficiency of frequency resource can be raised. However, the method using multiple carriers has a problem in that since multiple carriers having different frequencies are modulated and added, the variance of the envelope of transmitted signal is significantly large in comparison with the method using single carrier. Thus, to prevent the signal from being distorted, the electric power amplifier at the end of transmit system using multiple carriers must have significantly excellent linear characteristics in comparison with a single carrier system.
There are two methods suggested to reduce the variance of envelope of transmitted signal of multicarrier system. First method is to adjust the magnitude of baseband signal of each carrier by repeating Fourier Transform and Inverse Fourier Transform, and second method is to use a block coding.
In the first method, the magnitude of base band signal modulating each carrier is adjusted depending on the code selection of code and, as a result, the magnitude of signal for each channel is different. This makes frequency diversity gain reduced. This method has a disadvantage in the system complexity to apply the method to direct sequence spread spectrum communication. In the second method, a block coding technique should be used in code chip. Although this method can be applied to multicarrier direct sequence spread spectrum communication system, the new code generated as a result of block coding the PN code cannot be guaranteed to have the same property as a PN code for distinguishing from other user""s code, and the code generation apparatus becomes complicated.
Therefore, the object of the present invention is to provide a modulation apparatus of a multicarrier direct sequence spread spectrum communication system, which can reduce the envelope variation of transmitted signal by making a baseband signal have a sign opposite to the remaining three baseband signals among four baseband binary signals which modulate four different carriers.
A modulation apparatus of multicarrier direct sequence spread spectrum communication system according to a first embodiment of the present invention to achieve the above described object is characterized in that it comprises a pseudo noise code generation apparatus for generating a binary pseudo noise code; a spread spectrum combiner for generating a spread spectrum signal by EXOR operation of the binary pseudo noise code and user digital data; a waveform shaping apparatus for generating a shaped spread spectrum signal by shaping the waveform of the spread spectrum signal; a digital/analog converter for converting the waveform-shaped signal to analog value; a plurality of carrier modulation apparatuses for modulating a plurality of carriers each having different frequency by taking the analog values as inputs; and an inverter for converting the polarity of one input signal among the plurality of carrier modulation apparatuses.
Further, a modulation apparatus of multicarrier direct sequence spread spectrum communication system according to a second embodiment of the present invention is characterized in that it comprises a pseudo noise code generation apparatus for generating a binary pseudo noise code; a spread spectrum combiner for generating a spread spectrum signal by EXOR operation of the binary pseudo noise code and user digital data; a plurality of Walsh code generation apparatuses for generating Walsh sequence pairs having adjacent indexes; a plurality of signal combiners for generating a plurality of Walsh sequences; an inverter for inverting the polarity of one signal among a plurality of Walsh coded signals generated by the signal combiners; a plurality of waveform shaping apparatuses for generating a shaped spread spectrum signal by shaping the waveform of a plurality of walsh coded signals generated by the signal combiners and the waveform of the inverted Walsh coded signal generated by the inverter; and a plurality of digital/analog converters for converting the waveform shaped signal to analog values.